In recent years, weight reduction and cost reduction for various automobile parts have been implemented by forming these parts of a resin. Regarding an intake manifold for use in an automobile too, resin products have been the mainstream. Many of such intake manifolds made of a resin are fabricated by the vibration welding method (see Patent Document 1) comprising forming a flange at a rim of each one of a plurality of split members and joining these respective flanges by vibration welding.
As other methods for molding an intake manifold formed of a resin, there are such methods as the lost core injection molding method according to which a core having a low melting point is inserted into a mold and then this core is discharged therefrom by melting after resin molding process, a method according to which a hollow molded article is used as a core and a resin injection molding is effected on its outer side (see Patent Document 2, Patent Document 3).